Conventional valves, such as butterfly valves and gate valves, are not encountered by troubles when used as simple shut-off valves. However, when used for regulating flow rate under severe conditions, such as conditions for transporting high-pressure water, the conventional valves of this type are liable to generate drift currents and turbulence because the conventional valves, in general, have an irregular valve opening, and such drift currents and turbulence are liable to generate large noise and intense vibrations, to induce impulses, to cause rapid corrosion and breakage of the main component parts of the valves. On the other hand, lift valves are less liable to generate drift currents and turbulence because lift valves have a regular valve opening. However, the lift valves once closed are difficult to open due to unbalanced action of pressure on the valve element and the lift valves are unable to function properly if due attention is not paid. Therefore, it has been a usual practice to use a double-seated lift valve in which the axial thrust acting on the valve element is balanced or to use a single-seated valve having a spindle integrally provided with a balancing piston which reduces the axial thrust that acts on the valve element.
Nevertheless, the conventional lift valves still have many problems. As mentioned above, an axial thrust reducing means for a high-pressure lift valve or a large lift valve is complex and increases the manufacturing costs of the lift valve significantly, and it is very difficult to control flow rate in a pipe in which pressure varies violently. In lift valves, as well as in valves of other types, troubles due to the deposition of soil, sand and dirt are liable to occur when the lift valves are adjusted to low flow rate and these troubles are difficult problems to solve.
Accordingly, it is an object of the present invention to provide an easy-to-use controllable variable flow regulating lift valve solving those problems of the conventional lift valves, capable of exerting stable flow regulating performance, requiring very small power for valve operation, and provided with a flow regulating valve means and an automatic flow restriction means capable of an automatic cleaning function to avoid clogging.